Fortunately communication hasn't been a problem and we've talked about it. I learned that one before and it is excellent advice and a good reminder to those of us living life a little too fast sometimes.
Thanks also for the advice about meeting any others that come along. In this case she's also poly (he met her at a poly meeting, actually) so I know she has no problem with our relationship...but I have to admit that I find it a little intimidating since she already understands this part of him that I'm just now learning about. I've thought about this a lot but have landed on "not yet". I think you're right in that it probably has to do with my insecurities with the situation and I figure that will change some with time. Consider also that he and I have not been together that long - a handful of months, really. That said what we've got together is really great - we both joke that it's just been so EASY and I think because of that I'm not as insecure as I might have been - which has made working through this much easier for me. Well, that and I've always been accepting of where people find love. I love your quote "Love as thou wilt" (Kushiel...I've read them all). I have many gay friends and know one couple in a pretty open type relationship. I just never expected to find myself in anything other than "traditional". But then again, if he was a traditional guy I probably wouldn't like him so much...make sense?
I'm not sure where I'm going with this...so...I will think on your advice about meeting this other gal. I will probably eventually decide to do so - assuming that I am invited to and I probably will if I ask. I know I would like to attend one of his poly meetings and so I would meet her there most likely. He's invited me but I've not been able to get off work yet to attend - but I need to go for a number of reasons. Mostly for more enlightenment...I DO need to work towards understanding this fully if I'm going to make this work (because this is part of who he is) or decide (eventually) not to. I'm almost positive that I've landed in the former but occasionally my traditionally-raised brain says, "what the hell are you thinking? TWO is the right number...TWO..." but that just feels more and more "not right" lately.
So, LovingRadiance, thank you again for your time and feedback. I'm glad that you think that I'm on the right track with my thinking and I appreciate your advice.